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Interpreting storm surge height graphs

This plot (Figure 1) presents the long-term trend in skew surge from 1951-2099. Skew surge is the difference between the Total Water Level Elevation (TWLE), and the expected water level i.e. the additional elevation due the surge. The graph can be used to determine projections of change in storm surge height return frequency by a particular time period.

The red bars indicate that the climate change signal is evident when compared with the climate variability component. In the case of a white bar (not shown in this figure) it would indicate that the climate change signal is not distinct from the climate variability signal.

Each bar represents the range of the projections for storm surge return period, from the 5th percentile to the 95th percentile. The 50th percentile (central estimate) is represented by the black line in the centre of the bar.

Example of a storm surge height graph

Figure 1 long-term trend in skew surge from 1951-2099

Reading off the graph example: The possible level of storm (skew) surge for a particular location on the coast of the UK by the year 2050.

1. Start by calculating the time difference from now to the time period to be looked at (e.g. 2010 to 2050 = 40 years).

2. For this example, we choose to first look at the 95th percentile level for the return period we're interested in - say 20 years in this case (i.e. 1 in 20 year storm surge event).

3. We can immediately see that the climate change signal is evident as the bar is coloured red.

4. Then we read across from the top of the 20 year return period bar to the y axis and read off the result. This gives us the change in skew surge per year. The storm surge projections assume that the change is the same year to year into the future.

5. And finally, to make this value (0.7) relevant to the year 2050 we need to use the multiplier (40) we worked out earlier (40 years x 0.7mm/year = 28mm).